Several substituted 2-(2-pyridylmethyl)sulfinyl-1H-benzimidazole derivatives are well-known gastric proton pump inhibitors. These benzimidazole derivatives include lansoprazole, omeprazole, pantoprazole, and rabeprazole. This class of benzimidazole derivatives is generally represented by the following chemical formula A: 
U.S. Pat. No. 4,628,098 describes the generic lansoprazole compound. Lansoprazole has as its chemical name (2-[[[3-methyl-4-(2,2,2-trifluoro-ethoxy)-2-pyridinyl]methyl]sulfinyl]-1H-benzimidazole), i.e., when R1 is methyl, R2 is trifluoro-ethoxy, R3 is hydrogen and R4 is hydrogen.
As a characteristic shared with other benzimidazole derivatives (e.g., ompeprazole and pantoprazole), lansoprazole can inhibit gastric acid secretion, and thus commonly used as an antiulcer agent.
Several methods for preparing lansoprazole are known. The majority of these methods involve the use of a lansoprazole precursor that contains a thioether group. The thioether group is oxidized in the last step of preparation to form the lansoprazole.
U.S. Pat. Nos. 4,628,098 and 5,578,732 (the '732 patent) describe the oxidation of the thioether group using m-chloro-perbenzoic acid as the oxidizing agent. However, the use of m-chloro-perbenzoic acid often results in a non-selective oxidation of the thioether group. The '732 patent further describes an oxidation method with hydrogen peroxide (H2O2) in the presence of a specific vanadium catalyst. Other patents such as ES 2105953, WO 0121617, ES 2063705, U.S. Pat. No. 6,313,303, WO9947514, WO0168594 describe the use of other oxidation reagents and/or other catalysts. None of these oxidation methods result in selective oxidation of the thioether group.
In addition, the preparation of lansoprazole by conventional methods is always accompanied by the formation of small quantities of the corresponding sulfone derivative as an impurity. For example, U.S. Pat. No. 6,180,652 (the '652 patent) describes the presence of sulfone derivative. Formation of sulfone derivative brings about the drawback of low yield of the desired sulfoxide.
Although attempts have been made to separate the sulfone derivative from lansoprazole, it is not a simple task, given their very similar structures and physicochemical properties. For this purpose, the '652 patent describes a method which permits separation of lansprazole from its sulfone derivative. An acetone complex of the lansoprazole salt is purified in this method.
Lansoprazole and other 2-(2-pyridylmethyl)sulfinyl-benzimidazole derivatives tend to lose stability and undergo decomposition when contaminated with traces of a solvent, particularly water, in their crystal structure. It is desirable that the benzimidazole crystals be solvent free (i.e., residual solvent should be reduced to a minimum).
The '732 patent describes the crystallization of lansoprazole using a ethanol:water solvent system (vol:vol of ethanol:water is 9:1). U.S. Pat. No. 6,002,011 (the '011 patent) describes the crystallization of lansoprazole from the same ethanol:water system, containing traces of ammonia (0.03 mole NH4OH: 1 mole lansoprazole). The '011 patent discloses a reslurry method in water, which permits to obtain more stable “solvent free” lansoprazole. The '011 patent fails to disclose the level of purity for lansoprazole. In addition, the ethanol and water are difficult to eliminate. Even after intensive drying, lansoprazole still contains solvent and is unstable under storage.
There is continuing need to obtain 2-(2-pyridylmethyl)sulfinyl-1H-benzimidazoles (e.g., lansoprazole) that are free of contaminants including sulfone and sulfide derivatives. There has also been a long-felt need for a method to prepare 2-(2-pyridylmethyl)sulfinyl-1H-benzimidazoles (e.g., lansoprazole) having reduced water content (<0.1% wt/wt water).
We discovered that “solvent free” lansoprazole can be obtained by the crystallization from different solvents. Lansoprazole obtained by this method of crystallization can be dried to <0.1% water, as is required by the USP forum.